The Three-Dimensional Structure of Molecules - Isomerism

Isomerism

The problem which required the development of a structure theory was that some compounds have the same empirical formula but totally different properties. Dimethyl ether and ethyl alcohol, for example, have the same empirical formula C2H6O but behave very differently.

This appearance of different compounds with the same empirical formula is called isomerism.

The term isomerism had already been introduced in 1830 by Berzelius (from the Greek isos "identical" and meros "part").

For instance, from a combination even of a very limited number of atoms like 3 C, 6 H, and 1 O a variety of molecular structures can be generated, which differ in the sequence of the connections of the atoms.

Fig.1

Propionaldehyde, allyl alcohol, acetone, 2-methyloxirane and cyclopropyl alcohol all have the same empirical formula C3H6O, but they differ in their constitution. The isomerism can be subdivided into several categories.

If the different connectivity of the atoms of two or several isomers is the reason for their different physical and chemical properties, they are called constitutional isomers. The number of possible constitutional isomers rises steeply with more of atoms in the molecule. For example, this can be easily observed in the steep increase of the possible number of isomers of the alkanes.

If, however, the connectivity is the same but the spatial arrangement of the atoms is different, the isomers are called stereoisomers. A stereoisomer is described by the topology of the molecule and by its relation to other stereoisomers with the same constitution.

Two stereoisomers that have a mirror-image relationship and cannot be superimposed are called enantiomers. They are chiral. As an example to illustrate chirality one can take the left and the right hand which have the same mirror-image relationship. This is in fact, where the term chiral comes from (from the Greek cheir "hand"). Left and right hands cannot be superimposed and the right hand does not fit in the left glove.

Fig.2

Number of isomers of alkanes with increasing number of carbon atoms (CnH2n+2)

Isomers are subdivided into constitutional isomers and stereoisomers, and stereoisomers, in addition, can be differentiated as enantiomers and diastereomers.